A casing coupling is a short pipe used to connect two threaded casings. API couplings are also called
OCTG couplings and nipples. The internal threads are machined to match the external threads of the casing long joints. The two joints of the casing are screwed into the ends of the coupling, which is usually made of the same grade of steel as the casing to ensure its strength. All casing couplings are manufactured in accordance with the latest version of the API 5CT specification and are usually made seamlessly, with the same material grade as the pipe body (such as API 5CT K55/J55, N80, L80, P110, etc.). The PSL grade of the coupling is consistent with that of the casing, or a higher grade product is provided according to demand. Unless there are special requirements, the same heat treatment process is usually used.
Steel Coupling Definition
We know that a steel pipe coupling is a short pipe or tube with threads on both ends, which is used to connect pipelines. Therefore, it is a pipe fitting with threaded connection, and its advantage is that it is easy to repair the pipeline or replace the damaged pipeline.
The outer diameter of tubing coupling is generally up to 4 1/2". Since tubing is used in drilling operations (a sucker rod is installed inside the tubing for drilling/pumping), the coupling types are more diverse and strict than casing. The most commonly used is the thickened type thickened coupling (inner thickened and outer thickened), referred to as EUE coupling, which is the most popular coupling type in tubing connection. Tubing connection with EU end (EUE) coupling is also called UPTBG. Since the end of the tubing is thickened (thickened), the connection stress is better than the offset wedge type coupling.
The support joint is another type used for pipe connection, which has the same thickness as the pipe body, called NU end (NUE) pipe or TBG.
API pup joint specification
As we said, an important feature of API coupling is that it must be made of seamless pipe, and another feature is that it has the same grade as the casing or tubing body, and these materials are used to make blank couplings.
The coupling should be made from coupling blank or coupling blank, including hot forging. (Except C110 and Q125 grade couplings are not allowed to be hot forged. The materials of these two grades are usually different from other grades, so if you buy couplings of this grade, you should pay special attention to the provisions in the API 5CT specification. )
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API Tubing Coupling
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Descriptions and size
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Type of Thread
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O.D*L
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Weight
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(mm)
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(kg)
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Tubing Coupling
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1.05
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NUE
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33.35*80.96
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0.23
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1.05
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EUE
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42.16*82.55
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0.38
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1.315
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NUE
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42.16*82.55
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0.38
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1.315
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EUE
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48.26*88.90
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0.57
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1.66
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NUE
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52.17*88.90
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0.59
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1.66
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EUE
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58.88*95.25
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0.68
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1.9
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NUE
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55.88*95.25
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0.56
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1.9
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EUE
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63.50*98.42
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0.84
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2-3/8″
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NUE
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73.02*107.95
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1.28
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2-3/8″
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EUE
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77.80*132.82
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1.55
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2-7/8″
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NUE
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88.90*130.18
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2.34
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2-7/8″
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EUE
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93.17*133.35
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2.4
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3-1/2″
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NUE
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107.95*142.88
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3.71
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3-1/2″
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EUE
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114.30*146.05
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4.1
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4″
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NUE
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120.65*146.05
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4.35
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4″
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EUE
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127.00*152.40
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4.82
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4-1/2″
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NUE
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132.08*155.58
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4.89
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4-1/2″
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EUE
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141.30*158.75
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6.05
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API Casing Coupling
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Descriptions and size
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Type of Thread
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O.D*L
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Weight
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(mm)
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(kg)
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Casing Coupling
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4-1/2″
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STC
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127.00*158.75
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5.23
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4-1/2″
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LTC
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127.00*177.80
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4.15
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4-1/2″
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BTC
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127.00*225.42
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4.55
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5″
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STC
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141.30*165.10
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4.66
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5″
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LTC
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141.30*196.85
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5.75
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5″
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BTC
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141.30*231.78
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5.85
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5-1/2″
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STC
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153.67*171.45
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5.23
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5-1/2″
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LTC
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153.67*203.20
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6.42
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5-1/2″
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BTC
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153.67*234.95
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6.36
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6-5/8″
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STC
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187.71*184.15
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9.12
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6-5/8″
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LTC
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187.71*222.25
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11.34
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6-5/8″
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BTC
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187.71*244.48
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11.01
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7″
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STC
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194.46*184.15
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8.39
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7″
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LTC
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194.46*228.60
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10.83
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7″
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BTC
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194.46*254.00
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10.54
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7-5/8″
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STC
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215.90*190.50
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12.3
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7-5/8″
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LTC
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215.90*234.95
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15.63
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7-5/8″
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BTC
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215.90*263.52
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15.82
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8-5/8″
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STC
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244.48*196.85
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16.23
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8-5/8″
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LTC
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244.48*254.00
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21.67
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8-5/8″
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BTC
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244.48*269.88
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20.86
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9-5/8″
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STC
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269.88*196.85
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18.03
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9-5/8″
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LTC
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269.88*266.70
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25.45
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9-5/8″
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BTC
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269.88*269.88
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23.16
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10-3/4″
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STC
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298.45*203.20
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20.78
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10-3/4″
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BTC
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298.45*269.88
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25.74
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11-3/4′
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STC
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323.85*203.20
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22.64
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11-3/4′
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BTC
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323.85*269.88
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28.03
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13-3/8″
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STC
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365.12*203.20
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25.66
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13-3/8″
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BTC
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365.12*269.88
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31.77
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16″
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STC
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431.80*228.6
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34.91
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16″
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BTC
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431.80*269.88
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40.28
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18-5/8″
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STC
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508.00*228.60
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51.01
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18-5/8″
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BTC
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508.00*269.88
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62.68
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20″
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STC
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533.40*228.6
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43.42
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20″
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LTC
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533.4*292.10
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57.04
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20″
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BTC
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533.40*269.88
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50.1
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Difference between casing couplings and tubing couplings
There are casing couplings and tubing couplings, which have the same function of connecting pipes to each other, but there are still some differences between the two.
Tubing couplings
Usually, the maximum outer diameter of tubing couplings is 4 1/2", because tubing is used for drilling activities (the inside of the tubing is the sucker rod for drilling/pumping), and the coupling types are more diverse and strict than casing. The most commonly used is the thickened thickened coupling (inner thickening and outer thickening), referred to as EUE coupling, which is most suitable for tubing connections. Tubing connected with EU end (EUE) couplings is also called UPTBG. Due to the thickening (thickening) of the tubing end, the connection stress is better than the trapezoidal coupling.
The trapezoidal coupling is another type used for tubing connections. It has the same thickness as the pipe body and is called NU end (NUE) tubing or TBG.
Features
The present invention solves the problem of fatigue fracture of existing couplings due to stress concentration. The tubing end is connected to the inner wall of the coupling by a tapered thread, and the coupling body end and the tubing are connected by a flat thread with the same pitch. It is not easy to produce fatigue fracture, the connection effect is good, and it can effectively prevent the occurrence of oil well tubing fracture accidents.
Casing coupling
Casing coupling is suitable for pipes larger than 4 1/2 inches. Unlike tubing couplings, casing couplings usually do not require thickening (non-thickening), that is, there is no need to thicken casing couplings, so it has the following types:
• BTC: wedge coupling
• LTC: long wedge coupling
• STC: short wedge coupling
The three types of casing body have the same thickness, but different lengths.
The oil casing coupling is an essential accessory for connecting the upper and lower casings. The coupling manufacturing method is the same as that of seamless pipes. The steel pipe end has internal threads for connecting with the upper and lower casings. In order to ensure the sealing of the joint, strict requirements are placed on the accuracy of the threads.
API pup joint size
API casing and tubing joint sizes range from 1.05 inches to 20 inches (26.7 mm to 508 mm) in outer diameter, and the wall thickness usually ranges from 0.16 inches to 1 inch (4.2 mm to 22 mm).
Dimensions and tolerances
Several groups of coupling types are defined in API 5CT, as follows:
For groups 1, 2, and 3: Unless otherwise specified in the purchase agreement, the coupling dimensions and tolerances shall comply with the provisions of API 5CT Tables C.32 to C.35 or Tables E.32 to E.35.
For Group 4: The coupling can be internally processed, and external processing is also allowed. However, if other connecting couplings or threads (advanced connections) are used, the coupling size should be specified in the agreement. API couplings and threads must comply with C.32 and C.33 or E.32 and E.33.
Main Technical Parameters of Coupling
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Couplings
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Size in
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Max. OD in(mm)
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Min length in(mm)
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Grade
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NU
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EU
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NU
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EU
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23/8
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2.875(73.03)
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3.063(77.80)
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41/4(107.95)
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47/8(123.83)
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J55
N80
L80
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27/8
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3.500(88.90)
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3.668(93.20)
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51/8(130.18)
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51/4(133.35)
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31/2
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4.250(108.00)
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4.500(114.30)
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55/8(142.88)
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53/4(146.05)
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Crossovers
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All kinds of crossovers, connections and sub-couplings of grades J55, N80 and L80
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What is the use of API pup joint?
In simple terms, it is an internal threaded cylinder used to connect two sections of threaded pipe.
For this reason, API 5CT standard couplings play an important role in connecting casing and tubing line pipe. On the contrary, the biggest difference from general steel pipe couplings is that the manufacturing process of API couplings is more rigorous and requires precise control in the different processes listed below.
Raw material selection
Mechanical testing, including tensile strength
Impact testing
Chemical property control
Sulfide stress cracking test (for C90 or T95 material grade)
Dimensional tolerances, outer diameter, wall thickness, length and straightness
Thread control, number of threads per inch, thread angle, etc. (more stringent for advanced thread options)
Heat treatment
Additional markings
Material certification
Product specification level (PSL-2 or PSL-3)
Coupling Pipe Grade Identification By Paint Color
According to API 5CT standards, Oil casing and Tubing couplings should be painted one by one to distinguish different grades of steel. The color label of the oil casing and Tubing should be sprayed at any end ≥ 600mm, and the color should be painted on the whole outer surface, and then the color ring should be sprayed.
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Coupling colour code
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Grade
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Grade Type
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Colour(s) for Coupling
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Number andcolour of bandsfor product
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EntireCoupling
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Band(s)
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H40
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None
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Same as for pipe
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None/black band at the manufacturer’s option
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J55 Tubing
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Bright Green
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None
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One Bright Green
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J55 Casing
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Bright Green
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One White
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One Bright Green
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K55
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Bright Green
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None
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Two Bright Green
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M65
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M65Pipe uses L80Type 1Couplings
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One Bright GreenOne Blue
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N80
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1
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Red
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None
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One Red
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N80
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Q
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Red
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One Green
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One RedOne Bright Green
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R95
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Brown
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None
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One Brown
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L80
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1
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Red
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One Brown
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One RedOne Brown
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L80
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9Cr
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None
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Two Yellow
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One Red,One Brown,Two Yellow
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L80
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13Cr
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None
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One Yellow
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One Red,One Brown,One Yellow
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C90
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1
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Purple
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None
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One Purple
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T95
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1
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Silver
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None
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One Silver
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C110
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White
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Two Brown
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One White,Two Brown
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P110
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White
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None
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One White
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Q125
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Orange
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None
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One Orange
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How many types for the API couplings
There are several types covered in API 5CT standard for coupling specifications, classified in regular couplings, special clearance couplings, combination couplings, reducing couplings, seal ring couplings, and special bevel tubing regular couplings.
Regular couplings
Regular couplings diameters complied with API 5CT Tables C.32 to C.35 or Tables E.32 to E.35. Coupling bearing contact face inside and outside edges shall be rounded or broken, but shall not significantly reduce the width of the bearing face (dimension b), since less thickness will not be safe to support the pipe weight. The ends of couplings shall be faced exactly at right angles to the axis.
Special clearance couplings
In case specified in the contract, special-clearance (reduced outside diameter Wc) couplings for buttress (BTC connection) casing and external upset (EUE connection) tubing shall be furnished. Unless otherwise specified, special-clearance external upset tubing couplings shall have a special bevel on both ends. The inside and outside edges of the bearing face shall be rounded or beveled. The ends (or root face when beveled) of couplings shall be faced at right angles to the axis.
Special-clearance coupling dimensions and tolerances shall comply to as given in Tables C.33 and C.35 or Tables E.33 and E.35 and as shown on Figures D.3 and D.5. Color identification see clause 11.
Combination couplings
Combination couplings with different types of thread of the same specified size shall be furnished when specified on the contract. The minimum length and minimum OD of this coupling shall be sufficient to work with the specified size specified threads.
Reducing couplings
Reducing coupling is also kind of pup joint, used to connect two pipes of different diameter with the same or different types of thread
on the two ends. Same with combination coupling, minimum length and minimum diameter of reducing couplings shall be sufficient to support the specified size and threads.
Seal-ring couplings
Sealing-ring couplings conforming to the requirements shall be furnished when specified in the PO. Grooves may be cut before or after threading at manufacture’s option. Grooves and threads shall be free of fins, wickers and ribbons that are loose or can become loose and fold into the thread dorm. Couplings shall be inspected after final machining of the groove.
Special clearance coupling
When specified in the order, special-clearance (reduced outside diameter) couplings for buttress casing and external upset tubing shall be furnished.
Combination coupling
Reducing coupling are used to connect two pipes of different diameter with the same or different types of thread on the two ends, and shall be furnished when specified on the agreements. The minimum length and minimum diameter of reducing couplings shall be sufficient to accommodate the specified size and type of threads.
Other requirements in API 5CT standard
1. Appearance quality
According to
API 5CT specification, there shall be no folding, cracking, separation, scaling or other defects on the inner and outer surfaces of the coupling.
2. Test methods
The tests conducted for manufacturing couplings are similar to those conducted for manufacturing casing and tubing, including: tensile test, flattening test, hydrostatic pressure test, sulfide stress corrosion cracking test, grain size determination, transverse impact test and hardness test.
Baowi-steel provides API couplings for
casing and tubing connections. Please feel free to contact us for any requests regarding API or advanced connections.
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